The present invention relates to thin film heterojunction solar cells formed from compound semiconductors, especially copper indium diselenide (CuInSe.sub.2)-Cadmium Sulfide (CdS) solar cells and more particularly to an improved cell having a relatively thin n-type layer, e.g. cadmium sulfide, and a current collecting layer formed from a substantially conductive wide bandgap semiconductor material.
The background of copper indium selenide-cadmium sulfide solar cells is discussed in considerable detail in U.S. Pat. No. 4,335,266 issued to Mickelsen, et al. on June 15, 1982, which patent is hereby incorporated by reference for all purposes. As generally discussed in this reference, this type of solar cell has been seen by many as a viable alternative solar cell. The efficiency of such compound semiconductor solar cells has been gradually increased as improvements have been made. In addition they offer the opportunity of reduced manufacturing costs.
As taught in the Mickelsen patent, the cadmium sulfide layer, generally the n-type portion of the device, has been formed in two distinct regions. The first is a 0.5 to 1.5 micron layer of undoped cadmium sulfide which necessarily has a high resistivity. Superimposed on the high resistivity region is an indium doped layer of cadmium sulfide having a thickness of from 2 to 4 microns. This doped region has much higher conductivity and therefore forms an improved current collecting layer while providing the necessary electronic characeteristics to the cadmium sulfide layer near the p-n junction.
The basic improvement taught by Mickelsen involves a new method for forming the copper indium selenide layer so as to not have an adverse effect upon the cadmium sulfide layer when deposited. By thus avoiding degradation of the cadmium sulfide layer, Mickelsen provides an improved solar cell both in terms of efficiency and the reproducability which would be required for commercial production. However, for such devices to be competitive with known single crystal silicon devices and the existing thin film silicon devices, further efficiency improvements are needed.